1. Field of the Invention
The present invention relates to a displacement detecting apparatus which transforms a signal generated by in particular, a displacement quantity detector which adopts a phase modulation method, such as laser length measuring device and the like, into a numeric value which is proportional to the detected displacement quantity.
2. Description of the Prior Art
Generally, in the displacement detecting apparatus, such as a resolver, an inductosyn, a laser length measuring device and the like, in which a detected displacement quantity is outputted as a numeric value by phase-modulating an input reference wave with the measured displacement quantity, in order to detect the phase difference of the phase modulated wave from the reference wave, the quantity of the displacement is detected by counting the quantity of the phase difference based on a counting pulse which is synchronized with the reference wave and which has a frequency which is an integer multiple of that of the reference wave. However, although this method is useful for the resolver, the inductosyn or the like in which the reference wave can be formed by dividing the counting pulse, in such the detector as a laser length measuring device where the frequency of the reference wave is to be determined depending upon the intrinsic character of the laser oscillator, the counting pulse synchronized with the reference wave has been necessarily generated by means of a PLL (Phase Locked Loop) circuit. However the poor stability of the frequency of the reference wave in the laser oscillator has generated detecting errors caused by a response delay of the PLL circuit. Furthermore, such a displacement detecting apparatus must be provided with a high frequency counting pulse and its counter for obtaining a high resolution, thus exhibiting difficulties in improving the resolution.
FIG. 1 shows a constructional view of a laser length measuring device and a displacement detecting apparatus based on the conventional method. In this figure, a laser oscillator 1 generates two kinds of light beams f.sub.1 and f.sub.2 having different frequencies and different polarization planes. The combination of the outputted light beams f.sub.1 and f.sub.2 is optically divided by a beam splitter 2 into two paths, and then one path of the combined divided light beams f.sub.1 and f.sub.2 is converted by means of a photodetector 6 into an electric signal F.sub.R denoting the light-interference intensity for the light beams f.sub.1 and f.sub.2. Here, the frequency of the electric signal F.sub.R coincides with a difference in frequency between the light beams f.sub.1 and f.sub.2, and as is adopted as the reference wave F.sub.R (F.sub.R =.vertline.f.sub.1 -f.sub.2 .vertline.). The other path of the combined divided beams f.sub.1 and f.sub.2 is optically divided by another beam splitter 3 into two separate light beams f.sub. 1 and f.sub.2, the light beam f.sub.1 being directed to a movable reflecting mirror 5. When the light beam f.sub.1 is directed to the movable reflecting mirror 5 and when the movable reflecting mirror 5 is moving in the X-axis direction, the light beam f.sub.1 is affected by the doppler modulation of .+-..DELTA.f which is proportional to the moving speed of the reflecting mirror 5, and consequently the reflected light beam f.sub.1 .+-..DELTA.f is generated. On the other hand, the light beam f.sub.2 divided by the beam splitter 3 is reflected by a fixed reflecting mirror 4, and the reflected light beam f.sub.2 is combined with the reflected light beam f.sub.1 .+-..DELTA.f by the beam splitter 3. The mixed beams f.sub.1 .+-..DELTA.f and f.sub.2 are converted to an electric signal F.sub.P denoting the light interference intensity of light beams f.sub.1 .+-..DELTA.f and f.sub.2 by means of photodetector 7. Here, the electric signal F.sub.P has a frequency which corresponds to the frequency difference between the light beams f.sub.1 .+-..DELTA.f and f.sub.2. Accordingly, when the electric signal F.sub.R is adopted as the reference wave, the electric signal F.sub.P becomes a phase modulated wave which is formed by modulating in phase the reference signal F.sub.R by the shifted displacement X of the movable reflecting mirror 5. Concretely, if the wave length of the light beam f.sub.1 is denoted by .lambda., the phase modulated wave F.sub.P is to be displaced by a shift (4.pi./.lambda.)X.
The description above is directed to the basic principle for the laser length measuring device, and the operation of the displacement detecting apparatus of FIG. 1 will now be described with reference to the timing chart shown in FIGS. 2A to 2J.
Firstly, the reference wave F.sub.R outputted from the photodetector 6 is subjected to waveform shaping in a comparator 23 to form a signal DF.sub.R as shown in FIG. 2A, and the formed signal DF.sub.R is inputted into a phase comparing unit 9 to be compared with a most significant bit signal MSB (refer to FIG. 2B) of an outputted signal T' on a counter 8. Then, a voltage RV having a pulse width proportional to the phase shift is outputted. The voltage RV from the phase comparing unit 9 is smoothed by an LPF (low pass filter) 10, and inputted into a VCO (voltage controlling oscillator) 11 in order to oscillate and output a counting pulse CLK', as shown in FIG. 2C, having a frequency in accordance with the phase difference of the signal MSB from the signal DF.sub.R. Then the counting pulse CLK' is inputted into the counter 8 to be divided by "64", as is shown in FIG. 2D, to be adopted as the most significant signal MSB of the counter output signal T'. As detailed above, a PLL circuit is constructed of the phase comparing unit 9, the LPF 10, the VCO 11 and the counter 8, and generates the counting pulse CLK' to be approximately synchronized with the reference wave F.sub.R and to have 64 times the frequency of the reference wave F.sub.R. On the other hand, the output signal T' of the counter 8 is, as is shown in FIGS. 2A and 2D, formed to be a signal changing sawtoothwise from "0" to "63" within about one cycle of the reference wave F.sub.R. The output signal T' of the counter 8 is stored as a signal x' as shown in FIG. 2F in a latch circuit 12 when the signal DF.sub.P falls down which, as shown in FIG. 2E, has been waveform-shaped from the phase modulated wave F.sub.P by the comparator 17. The signal x' becomes the phase difference of the phase modulated wave F.sub.P to the reference wave F.sub.R within the range of the wave length of the reference wave. Employing the heretofore, it is possible to detect the displacement X of the movable reflecting mirror 5 with a precision of up to .lambda./2.
The signal x' is stored as a signal x" in a latch circuit 13 when the next falling transition of the phase modulated wave F.sub.P occurs. Consequently, the signal x" is adopted to precede the signal x' changing. The signals x' and x" are subjected to a subtraction operation in an updown pulse generator 14. The up pulse U.sub.P shown in FIG. 2H or down pluse D.sub.P is outputted based on the condition indicated by the following expression (1) in accordance with the subtraction value .DELTA.x as shown in FIG. 2G. ##EQU1## The up pulse U.sub.P and the down pulse D.sub.P from the up-down pulse generator 14 cause an up-down counter 15 to count up or down, respectively. The phase difference of the phase modulated wave F.sub.P from the reference wave F.sub.R is outputted in terms of the number x.sub.u of the wave length of the reference wave F.sub.R by way of the up-down counter 15 (refer to FIG. 2I). Here, a signal x having the signal x' as the lower digit and the signal x.sub.u as the upper digit, that is, the signal x of which value is 64.times.x.sub.u +x' becomes the phase difference x of the phase modulated wave F.sub.P from the reference wave F.sub.R. As a result, the shifted displacement X of the movable reflecting mirror 5 can be detected at a resolution of .lambda./128.
The broken line characteristic A indicated in FIGS. 2D, 2F and 2J shows the value where there is no follow-up delay caused by a PLL circuit, and the hatched portion B in FIG. 2J shows the error caused by the follow-up delay, i.e., the error of the displacement detection.
As shown, the conventional apparatus as shown in FIG. 1 is disadvantageously liable to yield a large displacement detection error as such depicted by the hatched portion B, compared with the case where there is no follow-up delay caused by a PLL circuit such as depicted by the signal x in the timing chart in FIG. 2J.
In the conventional example of FIG. 1, the frequency difference between oscillated light beams f.sub.1 and f.sub.2 of the laser oscillator 1, in other words, the frequency of the reference wave F.sub.R is apt to be varied or affected as shown in the timing charts of FIGS. 2A to 2J. For this reason, the PLL circuit having an LPF does not enable the frequency of the counting pulse CLK' to exactly synchronize with and follow up the frequency variations of the reference wave F.sub.R, thus exhibiting a displacement detection value error caused by the follow-up error. Furthermore, in order to obtain the quantity of the displacement detection at a high resolution, it is necessary to provide a high frequency counting pulse and its counter, and consequently, it is difficult to realize the higher resolution.